After nine months on the periphery of the Republican race, tonight’s debate in Mesa, Arizona, was Rick Santorum’s opportunity to show he deserved to be considered a frontrunner. But instead of using the occasion to build on the surge that led him to the top of the national polls, the former senator flopped as Mitt Romney and Ron Paul pounded him unmercifully from the start of the evening to its finish. By the end of the night, the grim look on his face betrayed the effect of having to explain his stands on issues such as earmarks, being a “team player” in the Senate and his support for Arlen Specter and “No Child Left Behind.” Whereas in previous debates, he had been on the attack pointing out Romney’s inconsistencies, in Mesa, it was his turn to be on the defensive.

Though Romney was far from brilliant and took his own lumps over his own hypocritical positions on earmarks and healthcare, there was little question he emerged the victor if only because Santorum came across as both long-winded and surly. If recent polls in Michigan showed the Pennsylvanian’s momentum was slowing, this debate may have put a period on his brief moment in the lead. A good night for Santorum might have helped put him over the top in Michigan and maybe even in Arizona next week and done irreparable harm to Romney’s hopes. But we may look back at this night and say this moment was not only when Santorum began to fade but also when Romney salted away the nomination.

Ironically, it was on his weakest point — his position on contraception — that Santorum sounded the strongest when he parried a question on the issue and made the point that promiscuity and the breakdown of the family was doing great damage to society. No one on the stage disagreed with him on that.

Yet that was overshadowed by the way Santorum found himself getting buried on his Senate record of voting for spending bills and earmarks. Romney’s attack on this was, as Santorum pointed out, deeply hypocritical since he relied on congressional earmarks to fund the 2002 Winter Olympics that he led. But whatever good he did with that retort was lost by his angry replies to attacks on his record, especially the way he went along with the Senate leadership on a number of issues. Santorum was clearly exasperated by having to defend himself in this manner and it showed. He discovered it is a lot harder to score points in a debate when you are wearing the bull’s eye on your back that goes with being in the lead.

Santorum’s failure once again should allow Romney to vault back into the lead. It will also give him the momentum that may allow him to hold onto Michigan after falling behind there.

Newt Gingrich was back in strong debate form and even managed to do so while avoiding joining in the gang tackle of Santorum. Ron Paul also had a strong night belaboring Santorum on government spending from a purist point of view though whatever advantage he gained in the battle to avoid last place was lost by his attempt to rationalize Iran’s nuclear quest at the same time as the other three Republicans were uniting to blast President Obama’s failure to stop the Islamist regime.

But the only real winner was Romney, who was repeatedly able to take down the man who is leading him in Michigan. Rick Santorum had one shot at solidifying his status as a frontrunner but failed. The ripple effect of this defeat will be felt in every state where he hoped to compete.